The present invention is generally in the field of methods and devices for surgical placement of implants, especially into bone.
Surgical implantation of devices such as screws, pins, and other medical implants into bone is frequently the only means to safely immobilize the bone. Typically, this is done by passing a probe through the cortical bone, the dense, hard bone on the outside of bony structures, and into the cancellous bone, the soft, compliant spongy bone on the inside of the bone.
As shown in FIG. 1, the relevant structures are the pedicles 12 and vertebral body 10. These structures are comprised of two types of bone: cortical 14 and cancellous 16. Cortical bone is the dense, hard bone covering the illustrated structures. Cancellous bone, commonly referred to as “spongy bone” is “soft” and compliant and provides the inner core for these structures.
Surgeons exploit the difference in these two bone types during pedicular cannulation. When passing a blunt, narrow “probe” through the pedicle, the instrument tip tends to follow the path of least resistance, the cancellous bone. The operator continues to direct this instrument, usually with x-ray assistance, until it has penetrated 50-80% of the anterior/posterior diameter of vertebral body. Successful cannulation is achieved when an intra-cancellous pilot channel is created without a breach of the cortical bone. A breach can injure critical structures in close proximity, such as spinal cord, nerve root, and vessels. The larger the cancellous inner core and the thicker the outer cortex, the easier the task. This is the case, for example, in the lumbar vertebrae, particularly the L3-S1 pedicles. However, in ascending the spine from the lumbar to thoracic and cervical vertebrae, the complexity of the task increases substantially. Since pedicular cannulation is essentially a “blind” technique, tactile feedback is critical to the operator during creation of the pilot channel. When the boundaries of the bone type are large and well defined, as they generally are in the lumbar pedicles, the relatively thick cortical wall and large core of cancellous bone facilitates intraosteal passage of a blunt tipped probe. The cortical/cancellous boundary is readily detected as the probe is advanced. In higher vertebrae, i.e., thoracic and cervical, the pedicle dimensions decrease markedly. As the overall cross-sectional diameter of the pedicle decreases, so does the cortical wall thickness. As the operator's tactile sensitivity to the cortical/cancellous boundary decreases, the risk for breach increases, even with adjunctive virtual image guidance.
A high complication rate associated with pedicle screw placement in lumbar vertebrae is well documented. As previously stated, the risk is even higher in thoracic and cervical spine. Placement of pedicle screws in the certical vertebrae, with the exception of perhaps C2 and C7, is virtually unheard of. Most posterior fixation procedures of the cervical spine, therefore, are through screw fixation in the lateral masses; not nearly as strong as pedicular fixation.
Since pedicular fixation in many cases provides for maximum construct stability and strength an alternative and improved method and mode of navigation is essential for routine cannulation of these upper vertebral pedicles.
Currently, there is no simple or reliable method to navigate cannulation of vertebral pedicles in vivo and in real time during placement of implants. This is a challenging task even in the hands of the most experienced spine surgeon, especially in the upper thoracic and cervical vertebrae. Current modes of virtual guidance are all based on “historical” data and the images upon which the guidance is dependent do not necessarily present the actual anatomic position at any given instant in real time an instrument is engaging tissue.
It is therefore an object of the present invention to provide methods and devices to guide cannulation or other procedures within bone or similar types of materials in the body, which are reliable and realtime.